Merriam-Webster Collegiate Dictionary - auxin

 noun  Etymology: International Scientific Vocabulary, from Greek auxein to increase — more at eke  Date: 1934  1. any of various usually acidic organic substances that promote cell elongation in plant shoots and usually regulate other growth processes (as root initiation): as  a. indoleacetic acid  b. any of various synthetic substances (as 2,4-D) resembling indoleacetic acid in activity and used especially in research and agriculture  2. plant hormone  • ~ic adjective